I think that has something to do with a misplaced belief/hope/desperation/rationale that we are more in control in the car. We are already on the ground.
In a plane, 20,000 feet in the air, once that catastrophe happens, you are either in the middle of an ocean, or a fireball in the side of mountain... We are so totally without means of effecting our fate that it becomes much more scary a situation.

Of course, as you said, the reality of the probability is much different, but the mind is more powerful.